From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 594922DC78D for ; Fri, 9 Jan 2026 17:32:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767979927; cv=none; b=PYMZQyRrzo0dnFe1UTAzUehhUgi74qYDCNzimes7b2HgR5JHEVH1NJ0IPxx8g+4NO+0oeJNsX88/RRA5PlcQ3HPpW1tZWpUfWx4JOqX7Qskf4wPVLMhkKvMB8yTKdDyB0Rg/Rz7Qo5c8L1cZ7NM6cqesEraDnVnrq5exKGZe04E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767979927; c=relaxed/simple; bh=NUMdQS00w2uiyTP712LRVWwLS5uwiQjstrFqaeib/HU=; h=Message-ID:Date:MIME-Version:Subject:To:References:Cc:From: In-Reply-To:Content-Type; b=qcOb8tlmHY1MTzdQzvHBocJC328sTzzMIyU1gAbYCrAETJTVRuSVvY1FXvJHrDvCCjCNmah5fvdX98BgsctQJ70R8UqFCqENliO/8lgWq8GgYtsuHEW9O7vD1SY3R1nhATjxU2vBLL35VPA2PHR5iG7jTMfucsrSv6wIBMuS+SU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kI42mGF7; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="kI42mGF7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AB23AC4CEF1; Fri, 9 Jan 2026 17:32:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767979926; bh=NUMdQS00w2uiyTP712LRVWwLS5uwiQjstrFqaeib/HU=; h=Date:Subject:To:References:Cc:From:In-Reply-To:From; b=kI42mGF7mAGgB1DUaM+HCHA+uJSf0Kwy4CJNgL+Ylroq6HKbf82uGgUy2ORKePL25 uz+erEJF/9Fm+bfY2dy3RwC8pD2jcAX1ynaDvuxqlUUdbQqOGLXubcaCx5Nr2f9lIx ux8TgPEY45Jx7xw4u57Xc2R/uprcqt1Z58+aYh6cuPr0/q7NHjo9U6Ta2QLiuB1pwH n3qCaDl+5CiU/K9CWP8Q6hK8/KkufrxMNzfDIqA9sdl1vONCN0jEV6jJnEhFfF+2g2 SXukyakydNz4q8Lhul1Wn8JA9ki60XdT5DEs9dNXxIyYpgfkyvMGBxkA4WcvkXeZCl OvlGbpEINqBLw== Message-ID: <685f4a41-b90c-4f8f-b4be-531eae1905ce@kernel.org> Date: Fri, 9 Jan 2026 11:32:05 -0600 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] drm/dp: Add byte-by-byte fallback for broken USB-C adapters To: "Chia-Lin Kao (AceLan)" References: <20251204024647.1462866-1-acelan.kao@canonical.com> Content-Language: en-US Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org From: Mario Limonciello In-Reply-To: <20251204024647.1462866-1-acelan.kao@canonical.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12/3/25 8:46 PM, Chia-Lin Kao (AceLan) wrote: > Some USB-C hubs and adapters have buggy firmware where multi-byte AUX > reads consistently timeout, while single-byte reads from the same address > work correctly. > > Known affected devices that exhibit this issue: > - Lenovo USB-C to VGA adapter (VIA VL817 chipset) > idVendor=17ef, idProduct=7217 > - Dell DA310 USB-C mobile adapter hub > idVendor=413c, idProduct=c010 > > Analysis of the failure pattern shows: > - Single-byte probes to 0xf0000 (LTTPR) succeed > - Single-byte probes to 0x00102 (TRAINING_AUX_RD_INTERVAL) succeed > - Multi-byte reads from 0x00000 (DPCD capabilities) timeout with -ETIMEDOUT > - Retrying does not help - the failure is consistent across all attempts > > The issue appears to be a firmware bug in the AUX transaction handling > that specifically affects multi-byte reads. > > Add a fallback mechanism in drm_dp_dpcd_read_data() that attempts > byte-by-byte reading when the normal multi-byte read fails. This > workaround only activates for adapters that fail the standard read path, > ensuring no impact on correctly functioning hardware. > > Tested with: > - Lenovo USB-C to VGA adapter (VIA VL817) - now works with fallback > - Dell DA310 USB-C hub - now works with fallback > - Dell/Analogix Slimport adapter - continues to work with normal path > > Signed-off-by: Chia-Lin Kao (AceLan) Reviewed-by: Mario Limonciello (AMD) As this fixes reads for some existing hardware on the market and is just in fallback path I feel this is low risk. I've applied this to drm-misc-fixes. a8f49a0043011 (HEAD -> drm-misc-fixes) drm/dp: Add byte-by-byte fallback for broken USB-C adapters > --- > v2. 1. Move the workaround from intel_dp_read_dprx_caps() to > drm_dp_dpcd_read_data(), so that it applies to all DPCD reads across > all DRM drivers benefit from this fix, not just i915. > 2. Move the definition of drm_dp_dpcd_readb() before > drm_dp_dpcd_read_data() > --- > include/drm/display/drm_dp_helper.h | 57 +++++++++++++++++++---------- > 1 file changed, 37 insertions(+), 20 deletions(-) > > diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h > index df2f24b950e4..14d2859f0bda 100644 > --- a/include/drm/display/drm_dp_helper.h > +++ b/include/drm/display/drm_dp_helper.h > @@ -551,6 +551,22 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset, > ssize_t drm_dp_dpcd_write(struct drm_dp_aux *aux, unsigned int offset, > void *buffer, size_t size); > > +/** > + * drm_dp_dpcd_readb() - read a single byte from the DPCD > + * @aux: DisplayPort AUX channel > + * @offset: address of the register to read > + * @valuep: location where the value of the register will be stored > + * > + * Returns the number of bytes transferred (1) on success, or a negative > + * error code on failure. In most of the cases you should be using > + * drm_dp_dpcd_read_byte() instead. > + */ > +static inline ssize_t drm_dp_dpcd_readb(struct drm_dp_aux *aux, > + unsigned int offset, u8 *valuep) > +{ > + return drm_dp_dpcd_read(aux, offset, valuep, 1); > +} > + > /** > * drm_dp_dpcd_read_data() - read a series of bytes from the DPCD > * @aux: DisplayPort AUX channel (SST or MST) > @@ -570,12 +586,29 @@ static inline int drm_dp_dpcd_read_data(struct drm_dp_aux *aux, > void *buffer, size_t size) > { > int ret; > + size_t i; > + u8 *buf = buffer; > > ret = drm_dp_dpcd_read(aux, offset, buffer, size); > - if (ret < 0) > - return ret; > - if (ret < size) > - return -EPROTO; > + if (ret >= 0) { > + if (ret < size) > + return -EPROTO; > + return 0; > + } > + > + /* > + * Workaround for USB-C hubs/adapters with buggy firmware that fail > + * multi-byte AUX reads but work with single-byte reads. > + * Known affected devices: > + * - Lenovo USB-C to VGA adapter (VIA VL817, idVendor=17ef, idProduct=7217) > + * - Dell DA310 USB-C hub (idVendor=413c, idProduct=c010) > + * Attempt byte-by-byte reading as a fallback. > + */ > + for (i = 0; i < size; i++) { > + ret = drm_dp_dpcd_readb(aux, offset + i, &buf[i]); > + if (ret < 0) > + return ret; > + } > > return 0; > } > @@ -609,22 +642,6 @@ static inline int drm_dp_dpcd_write_data(struct drm_dp_aux *aux, > return 0; > } > > -/** > - * drm_dp_dpcd_readb() - read a single byte from the DPCD > - * @aux: DisplayPort AUX channel > - * @offset: address of the register to read > - * @valuep: location where the value of the register will be stored > - * > - * Returns the number of bytes transferred (1) on success, or a negative > - * error code on failure. In most of the cases you should be using > - * drm_dp_dpcd_read_byte() instead. > - */ > -static inline ssize_t drm_dp_dpcd_readb(struct drm_dp_aux *aux, > - unsigned int offset, u8 *valuep) > -{ > - return drm_dp_dpcd_read(aux, offset, valuep, 1); > -} > - > /** > * drm_dp_dpcd_writeb() - write a single byte to the DPCD > * @aux: DisplayPort AUX channel